What happened to Mars' atmosphere and water has been a well guarded cosmic mystery.
One idea was that the martian atmosphere was "eroded" over billion years by powerful solar winds. A more controversial idea is that Mars' atmosphere was blown into space by a catastrophic asteroid or comet impact in the remote past. A space rock at least 6 miles (10 kilometers) wide could have been enough to obliterate Mars's once abundant atmosphere. However, scientists may "overplay" the impact hand when explaining planetary change.
An urgent note: NASA's founding space mission as well as the future of space science research in the United States is under assault by the Obama administration. If you believe in this nation's leadership role in space, please contact the White House, your senators and congressman, and let them know that you want NASA to receive the federal funding it deserves for ongoing space research and the human exploration of space.
What's in the Sky: The crescent Moon, as thin as a fingernail, rests on its back at dusk Feb. 14-17. On Feb. 25, the nearly full Moon will join Mars in the evening sky.
Former NASA Ames Research Center science writer Lou Varricchio. M.Sc., lives in Vermont. He is currently the NASA/JPL solar system ambassador in Vermont. He is available, free of charge, for classroom presentations and public talks. He is also a NASA resource to area science teachers. E-mail him at firstname.lastname@example.org.